Paper product and method of making same



Patented May 7, 1946 PAPER. PaonUc'r aNn METHOD OF MAKING sans Kennethw. Britt, Norwood, Pa., mightito Scott Paper Company, Chester, Pa acorporation of Pennsylvania No Drawing. Application August 13, 1941,

Serial No. 406,128

11 Claims. (or 92-49) The present inventionrelates to paper havingadvantageous properties, and to the method of making the same. By thisinvention, a' paper may be prepared in which desired characteristics,depending on the intended use of the paper, may be imparted thereto by asimple and inexpensive method. i

In the usual paper-making process, 'cellulosic fibres, i. e., fibresderived from various plant materials and consisting essentially, but notentirely, of cellulose, are deposited from suspension in water toproduce an interfelted web. A considerable amount of adhesion or bondingbetween the individual fibres in contact with each other develops upondrying this web, and it is this peculiar property of cellulosic fibrethat accounts for the strength of paper. One of the outstandingcharacteristics, however, of the'normal interfibre adhesion in a sheetof paper is the loss of most of this adhesive strength when the sheet ofpaper becomes wetted with water. Thus, the usual absorbent paper retainsonly 5% to 10% of its dry strength, when wetted with water. The lack ofwet strength is disadvantageous in the various types of paper, but isparticularly marked in the case of products which during their use comeinto contactwith moisture, for example, it is especially disadvantageousin a papertowel, the normal use of which takes place in the wet orpartially wet state. In such a product. it is, of course, essential thatits characteristics, such as absorbency, porosity and softness beretained, for otherwise the product would not serve satisfactorily forits intended purpose.

The principal object of the present invention is to provide a paper, inwhich the interfibre adhesion or cementation in the sheet has beenmodmetal oxide associated with silica) .within and upon the surfaceofthe fibres, said alkali metal silicate being distributed throughoutthe interfelted structure of the paper. The increase in wet-strength maybe due to the presence of the alkali metal silicate alone, or to thepresence of the alkali metal silicate together with silica ,derivedtherefrom. The purpose of converting a portion of, the alkali metalsilicate into silica is to provide a product with reduced alkalinity.

The product, is prepared byimpregnatingpaper with a solution of analkali metal silicate with or without treatment with a substance causingthe conversion of a portion of the alkali metal silicate intosilicicacid during the process, and by heating the impregnated paper to depositthe alkali metal silicate or the alkali metal silicate and silica withinandupon the surface of the fibres throughout the interfelted structureof the paper.

The paper treated in accordance with the invention may be of any type orgrade, for ex- 7 ample, towel, waterleaf, kraft, bond, wall board,

building paper, outdoor advertising paper, photographic paper, wrappingpaper, and the like. As previously stated, the invention is particularlyapplicable for the treatment of towel paper and other papers, the normaluse of whichtakes place in the wet or partially wet state.

The present invention is to be differentiated I suspension in thebeater, followed by the addition of alum. The alum and sodium silicatereact to ifled so thatsaid bonding retains an appreciably greater degreeof strength when wetted.

A further object of the invention is to provide a paper having arelatively high degree of wet-strength without substantial impairment ofsuch properties as water absorbency and softness required in certainclasses of paper, which paper may be easily and economically preparedwithout the use of any toxic or other chemical disadvantageous in theintended use of the paper.

Other objects, including the provision of a novel method of preparingthe product of the invention, will be apparent from a consideration ofthe specification and claims.

In accidance with the present invention, a paper having wet-strength inexcess of that inher'ent in the normal paper structure, and otherimproved properties, is provided in which the increase in wet strengthis imparted to the paper by the presence of an alkali metal silicate(alkali form a precipitate, a portion of which adheres to the surface ofthe fibres, without penetration thereof. Injaccordance with the presentinvention; the paper after its ,formation into an interfelted webistreated with the alkali metal silicate solution which is absorbed by thefibres themselves, so that upon drying of the product, alkali metalsilicate, or alkali metal silicate together with silica, is depositedwithin, as well as upon the surface of the fibres, reducing theirtendency to swell upon the application of water.

As previously stated, he paper, either before or after drying, istreated with the alkali metal silicate solution. Advantageously, thesolution is applied to the paper while the paper sheet is beingprocessed, and the solution may be brought into, contact with the papersheet at any stage, after the formation of the interfelted fibre web andprior to the completion of the drying, where sufilcient water has beenremoved to permit absorption of the solution by the sheet. Preferably,the treatment with the solution takes place when the moisturein thesheet has been reduced to about 30% or below, and, therefore, as thepaper sheet leaves the intermediate drlers. The passage of the papersheet over the remaining drier section will be sufllcient to dry thepaper and to deposit alkali metal silicate, or the alkali metal silicateand silica, in at least a partially dehydrated form within and upon thesurface of the fibres, so that the tendency of paper fibres to swellupon the application of water will be reduced. In the case a dried papersheet is treated with the solution, the sheet'may be dried in anydesired manner by subjecting it to a temperature above 212 F. and belowthe temperature at which the paper is damaged, for example, at 250 F. to300 F. in a drying chamber or by passing it over a heated roll orthrough a heated calender.

The paper sheet, whether treated during the paper making process orafter drying, may be brought into contact with the alkali metal silicatesolution by any suitable means, in order to impregnate the paper. Forexample, it may be applied by means of a size press such as is used inthe conventional tub sizing process. Alternatively, it may be sprayedupon the moving sheet of the paper by means of a sprayin device.

The solution has been designated herein as an alkali metal silicatesolution which includes both potassium and sodium silicate, but sincethe sodium silicate solutions are more economical for use, the inventionwill be described hereinafter using sodium silicate solutions astypical. While a solution of any desired ratio of Nero to S102, wherethe molecular proportion of SiO: exceeds that of NazO, for example,solutions having a molecular ratio of NaaO to S102 between about 1 to3.8 and 1 to 2, may be employed, the use of a solution relatively low inalkalinity is preferred, since it is thus possible to associate a largeramount of sodium silicate with the paper without raising the alkalinityof the paper to an undesirable point. For example, a solution containinga molecular ratio of NaaO to SiOa of 1 to 3.8 or 1 to 3.3, suchsolutions now being available in the market, will usually be employed.If desired, the solution, however, may be prepared by dissolving solidsoluble sodium silicate in the appropriate amount of water, or a sodiumsilicate solution having-relatively high alkalinity may be adjusted tothe desired alkalinity by the addition of appropriate acidic material.The sodium silicate concentration in the solution is relativelyunimportant so long as the solution is sufficiently dilute to beabsorbed by the paper, an example of a range of concentrations which maybe employed being from .5 to preferably from 2% to 4%, although it is tobe understood that solutions of other concentrations may be used. In thetreatment of paper where it is desired to maintain a light color, theconcentration of the solution of alkali metal silicate should be belowthat point where appreciable discoloration, due to excessive alkali, isencountered. In the case of paper of the type treated in Example IV, ithas been found that concentrations above about 4% tend to causediscoloration.

The amount of sodium silicate, or of sodium silicate and silica,deposited within and upon and preferably will not exceed about 1.6%, figured as SiO2, by weight based on the dry weight of the paper. Thepresence of more than about 2.5% of sodium silicate (figured as SiOa)tends to produce undesirable properties in the paper, such asdiscoloration, odor and causticity. The presence of silica with sodiumsilicate produces an improvement in certain paper properties whichcannot be obtained with sodium silicate alone, due to excessivealkalinity. In the event that silica is present with the sodiumsilicate, the amount thereof is preferably equal to or less than that ofthe sodium silicate, but the amount of silica may, if desired, exceedthat of the sodium silicate. However, due to the fact that desirablecharacteristics of the paper, such as the dry-tear resistance andelasticity, tend to be decreased when the total sodium silicate andsilica present in the finished paper exceeds about 8% (figured as S102),the upper limit of the sodium silicate and silica present in the paper,in accordance with the present invention, is about 8% (figured as SiOn)by weight based on the dry weight of the paper. The amount of sodiumsilicate and silica employed in any particular instance will depend uponthe paper treated and the properties desired in the finished product.Herein, and in the claims, the term heat-treated material selected fromthe group consisting of an alkali metal silicate and a mixture of analkali metal silicate and silica refers to the sodium silicate and/orthe mixture of sodium silicate and the silica derived therefrom presentin the paper, in accordance with the present invention.

In the treatment of towel paper to increase its wet strength andresistance to rubbing, when wet, it is essential to obtain the normalcharacteristics of the paper, such as softness, for otherwise theproduct would not serve satisfactorily for its intended purpose.Therefore, the upper limit of sodium silicate, or of sodium silicate andsilica, will be controlled, so that the paper is not appreciably alteredin such characteristics, and the amount thereof will generally not beabove about 3.0% (figured as SiOz) by weight based on the dry weight ofthe paper, preferably between 0.5% and 2.0%, generally about 1% (figuredas S102). Advantageously, the heat treated material is made up ofapproximately equal parts of sodium silicate and silica. As willhereinafter appear, if in any particular case undesirable stiffness isimparted to the sheet by reason of the presence of the siliceousmaterial therein, a dispersible oil may be incorporated in the productto impart softness thereto.

Several methods are available for accomplishing the conversion of aportion of the sodium silicate into silica, it being understood that inaccordance with this invention there is always present at least 0.1%sodium silicate in the paper. For example, an acidic material may beadded to the sodium silicate solution in suftlcient amounts to result inthe deposition of silica, together with the sodiumsilicate, upon thedrying of the paper. If this method is followed, the acidic materialshould not cause the formation of a precipitate in the impregnatingsolution. Another method resides in the subjection of the p r, after impregnation, with the sodium silica e solution, to treatment with anacidic material, the amount of acidic material employed beinginsufficient to neutralize all of the sodium silicate. Preferably, thetreatment with acid follows the treatment with the silicate solutionwithout any intermediate drying step. If desired, however, the paper,

silicate withinand upon the surface of the fibres may be treated with anacidic material to neutralize a portion of the sodium silicate. In themethods involving the treatment with acidic material, such material maybe an inorganic or organic acid, or an acid salt. For example,hydrochloric, sulphuric, nitric, phosphoric, 'acetic, formic, oxalic orlactic acid, or sodium bisulphate. Mixtures of acids may be employed ifdesired.

Another method of, providing silica with the sodium silicate within andupon the surfaces of the fibres involves the incorporation-of an am- 1monium salt in the paper prior to the drying thereof. The ammonium salt,such as diammonium phosphate, ammonium chloride or ammoniumsulphate, maybe present inthe sodium silicate. solution and thus absorbed by thepaper with the sodium silicate. The presence of the ammonium saltresults in reduced alkalinity of the paper treated with the silicate dueto the volatilization of ammonia during the drying operation, withconsequent conversion of a portion of the silicate into silica. Insteadof using the ammonium salt in the solution of sodium silicate, the papermay, if desired, be treatedwith asolution 'of the ammonium salt eitherbefore or after its treatment with the but this method is not preferred,since it has no advantages over the use of a solution containing boththe sodium silicate and the ammonium salt. Generally, it is notpracticable to produce a paper containing more than about 5 use ofammonium salts.

When an ammonium salt is used with sodium silicate, in order to reducethe alkalinity of the finished product, the amount of such ammonium saltemployed is less than that necessary to completely neutralize the sodiumsilicate, i. e., the number of ammonium radicals so introduced is lessthan the sodium atoms contained in the so:

dium silicate. Accordingly, the weight of the ammonium salt inproportion to the sodium silicate will depend upon the alkalinity of thesilicate and upon the particular ammonium salt employed as illustratedby the following tables. The treated paper, therefore, will contain somesoda (NazO) still associated with silica.

Table I Column A, Column B, ratio of ratio of Diammonium phosphate percent sodium sodium of silicate by weight atoms to atoms to ammoniumammonium radicals radicals 100:22 100:!) 100: 100:40 100:66 lCllzGO100:88 -l :80 100:110 2100 Column A, Column B,

ratio of ratio of Ammonium chloride, per cent 1! sodium sodium silicateby weight atoms to atoms to ammonium ammonium i s radicals sodiumsilicate solution,

% free silica by the combination with sodium silicate of said 1-3.8

ratio will be less than about 45% by weight of the sodium silicate, andwith sodium silicate of said'l3;3 ratio, the, amount will be less thanabout 50%. In the case of ammonium chloride. the amount of ammonium saltfor the two kinds of silicate above referred to will be less than about37% and about 42%, respectively.

Another material that may be employed with the sodium silicatecontaining paper, irrespective of the presence of the ammonium salt, isa dispersible oil, for example, of the type known as a textile finishingoil. The term dispersible oil" as used herein and in theclaims includesthose oils which, either through their emulsiflable or solubleproperties, .are capableof being dispersed in an aqueous medium. Theseoils produce a softening efl'ect on paper treated with sodium silicatesolution, which eflect is not encountered to the same extent with paperwhich has not been treated'with the sodium silicate solution. Since theuse of the dispersible oil tends to cause a decrease in wet strength,the amount thereof associated with the paper will be kept below thepoint where the increase in wet-strength imparted by the sodium silicatewill be appreciably reduced by the oil. Satisfactory results areobtained by using about .l% to .5% of oil by weight based on the dryweight of the paper, and the use of amounts higher than this and up toabout 3%, or even higher, may be used if desired. The dispersible oilmay be applied to the paper, either by dispersing it inthe sodiumsilicate solution (the preferred mode of 'operation) or by treating thepaper with a solution of the dispersible oil either before or after itstreatment with the sodium silicate solution.

Since the product of the present invention contains sodium silicate, itis to be distinguished from the product of my copending application,Serial No. 406,727, filed even date herewith,in1which silica, butnosodium silicate. is deposited within and upon the surface of the fibresof the interfelted structure.

The following examples are illustrative of the I 0 per ream preparedfrom 50% bleached sulphate and 50% unbleached sulphite pulps is treatedwith a 2% solution of sodium silicate (3.8 $101450 1 NaaOl, the amountof solution applied being 40% of the weight of the paper treated. Thepaper is dried by heating -it to a temperature in the neighborhood of275 F., and contains 0.8% sodium silicate (dry basis). The wet tensilestrength of the untreated paper is 0.25. pound as compared to 0.75 poundfor the processed paper.

. Example 11 An air-dried towel Example I is treatedwith a solutioncontaining i 3% sodium silicate (as $101 to 1 N820), and 1%Bocony-Vacuum Company's "Textile finishing oil paper of the'grade sed insodium silicate and 0.2% dispersible oil (dry basis). The wet tensilestrength oi! the untreated paper is 0.25 pound, as compared to 0.50pound for the processed paper.

Example III An air-dried towel paper of the grade used in Example I istreated with a solution containing 4% sodium silicate (3.8 SiOa to 1NazO) and 1% diammonium phosphate, the amount of solution applied being30% of the weight of the paper treated. The paper is dried by heating itto a temperature of about 250 F. and contains 54% sodium silicate, 53%silica (.96% sodium silicate and silica figured as SiOz) and .3% ofphosphate figured as diammonium phosphate (all on a dry basis). The wettensile strength oi the untreated paper is 0.25 pound, as compared to1.00 pound for the processed paper.

Example IV 100 pounds of towel paper (dry basis) weighing 34 pounds perream prepared irom 65% unbleached sulphite and 35% mechanical woodpulps, containing 30 pounds of water as it travels between theintermediate and final drier sections is passed through a solutioncontaining 4% sodium silicate (3.8 NazO to 1 S102), 0.6% diammoniumphosphate, and .2% dispersible oil used in Example II, the amount ofsolution applied being 20 pounds. The paper is dried by passing it overthe final drier section at the usual temperature maintained therein andcontains 0.54% sodium silicate, .2l% Silica (.63% sodium silicate andsilica figured as SiOz), 0.12% phosphate figured as diammcniumphosphate, and 0.04% dispersible oil. The wet tensile strength of theuntreated paper is 0.25 pound, as compared to .75 pound for theprocessed paper.

The properties of the product of the present invention may be furthermodified by the incorporation therein of other materials, such asstarch, glue and vegetable gums.

Considerable modification is possible in the methods of processing, aswell as in the amounts of alkali metal silicate and other materials (ifany) incorporated in the paper without departing from the essentialfeatures of my invention.

I claim:

1. A paper having increased wet-strength and substantially unimpaireddry tear resistance, and having heat-treated material selected from thegroup consisting of an alkali metal silicate and a mixture of an alkalimetal silicate and silica, reducing the tendency of the paper fibres toswell upon the application of water, within and upon the surface of thefibres throughout the interfelted structure of said paper, saidheattreated masaid paper and present in an amount between .01% and about2.5% figured as SiOa, by weight based on the dry weight of the paper,said sodium silicate being the only siliceous material present insufficient quantity to contribute materially to the wet-strength of thepaper.

3. A paper having increased wet-strength and substantially unimpaireddry tear resistance. and

terial being present in an amount between 0.1%

and about 8%, figured as SiOz, by weight based on the dry weight or thepaper, at least a portion of said heat-treated material being alkalimetal silicate present in an amount not exceeding about 2.5%, figured asSiOz basd on the dry weight of the paper.

2. A paper having increased wet-strength and substantially unimpaireddry tear resistance, and having heat-treated sodium silicate reducingthe tendency of the paper fibres to swell upon'the application of water,within and upon the surface of the fibres throughout the interfeltedstructure of having heat-treated sodium silicate and silica reducing thetendency of the paper fibres to swell upon the application of water,within and upon the surface of the fibres throughout the interfeltedstructure of the paper and present in an amount less than about 8%,figured as S102, by weight based on the dry weight of the paper, thesodium silicate being present in an amount be-- tween 0.1% and about2.5%, figured as S102, by weight based on the dry weight of the paper.

4. A towel paper having increased wet-strength and substantiallyunimpaired dry tear resistance. and having heat-treated sodium silicateand silica reducing the tendency of the paper fibres to swell upon theapplication of water, within and upon the surface of the fibresthroughout the interfelted structure of the paper and present in anamount between approximately .5% and 2.0%, figured as S102, by weightbased on the dry weight of the paper, the sodium silicate being presentin an amount between 0.1% and about 1%, figured as SiOz, by weight basedon the dry weight of the paper,

5. The method of increasing the wet-strength of paper while preservingthe dry tear resistance thereof, which comprises associating within andupon the surface of the fibres throughout the interfelted structure ofsaid paper a heat-treated material selected from the group consisting ofan alkali metal silicate and a mixture of an alkali metal silicate andsilica, reducing the tendency of the paper fibres to swell upon theapplication of water, in an amount between 0.1% and about 8%, figured asS102, by weight based on the dry weight of the paper, at least a portionof said siliceous material being alkali metal silicate in an amount notexceeding about 2.5%, figured as S102, by weight based on the dry weightof the paper.

6. The method of increasing the wet strength of paper while preservingthe dry tear resistance thereof, which comprises impregnating paper witha solution of sodium silicate in an amount sufllcient to furnish to thepaper between 0.1% and about 2.5% sodium silicate, figured as S102, byweight based on the dry weight of the paper, and heating said paperabove 212 F. to deposit sodium silicate reducing the tendency of thepaper fibres to swell upon the application of water, within and upon thesurface of the fibres throughout the interfelted structure of the paper.

7. The method of increasing the wet strength of paper while preservingthe dry tear resistance thereof, which comprises impregnating paper witha solution of sodium silicate in an amount suflicient to furnish to thepaper between 0.1% and about 2.5% sodium silicate, figured as SiOz, byweight based on the dry weight of the paper at a point in the productionof said paper after the formation of the interfelted web and prior tothe completion of the drying thereof, and heating said paper above 212F. to deposit sodium silicate reducing the tendency of the paper fibresto swell upon the application of water, within and upon the surface ofthe fibres throughout the interfelted structure of the paper.

8. The method or increasing the wet strength of paper while preservingthe dry tear resistance thereof, which comprises impregnating paper witha solution of sodium silicate in an amount sufiicient to furnish to thepaper between 0.1% and about 8% siliceous material, figured as S101, byweight based on the dry weight of the paper, neutralizing a portion ofthe sodium silicate absorbed by the paper to convert it into silicicacid and to provide between 0.1% and about 2.5% sodium silicate, figuredas $102, by weight based on the dry weight of the paper, and heatingsaid paper above 212 F. to deposit sodium silicate and silica reducingthe tendency of the paper fibres to swell upon the application of water,within and upon the surface of the fibres throughout the interfeltedstructure of the paper.

9. The method of increasing the wet strength of paper while preservingthe dry tear resistance thereof, which comprises impregnating paper witha solution of sodium silicate in an amount sufficient to furnish to thepaper between 0.1% and about 8% siliceous material, figured as S102, byweight based on the dry weight of the paper, neutralizing a portion ofthesodium silicate absorbed by the paper to convert it into silicic acidand to provide between 0.1% and about 2.5% sodium silicate, figured asSiOz, by weight oased on the dry weight of the paper, said steps beingconducted at a point in the production ofsaid paper after the formationof the interfelted web and prior to the completion of the dryingthereof, and heating said paper above 212 F. to dry it and to depositsodium silicate and silica reducing the tendency of the paper fibres toswell upon the application of water, within and upon the surface of thefibres throughout the interfelted structure of the paper.

10. The method of increasing the wet strength of paper while preservingthe dry tear resistance thereof, which comprises impregnating paper witha solution of sodium silicate in an amount sufilcient to furnish between0.1% and about 8% siliceous material, figured as SiOz, by weight basedon the dry weight of the paper and with a solution of an ammonium saltin an amount to convert a sufficient portion of said sodium silicateinto silica upon heating to provide between 0.1% and about 2.5% sodiumsilicate, figured as SiOz, by weight based on the dry weight of thepaper, and heating said paper above 212 F. to cause said conversion andto deposit sodium silicate and silica reducing the tendency of the paperfibres .to swell upon the application of water, within and upon thesurface of the fibres throughout the interfelted structure of the paper.

11. The method of increasing the wet strength of paper while preservingthe dry tear resistance and about 2.5% sodium silicate, figured as SiOz,

by weight based on the dry weight of the paper,

. said impregnation being conducted at a point in the production of saidpaper after the formation of an interfelted web and prior to completionof the drying thereof, and heating said paper above 212 F. to dry it andto cause said conversion to deposit sodium silicate and silica reducingthe tendency of the paper fibres to swell upon the application of water,within and upon the surface of the fibres throughout theinterfelted'structure of the paper.

KENNETH W. BRI'I'I.

